1. Field of the Invention
The present invention relates to a polishing apparatus and method for adjusting a surface of a polishing pad. It specifically relates to a polishing apparatus, where the surface of the polishing pad is polished so as to adjust the surface. The present invention can be used, for example, in the chemical mechanical polishing (CMP) process, through which the surface of an object, such as a silicon wafer is polished.
2. Description of the Related Art
Within the ordinary process steps of manufacturing a semiconductor device, an active device such as a MOS transistor is formed on a silicon substrate; and an aluminum interconnect is patterned and formed over the surface of the substrate. Therefore, the surface of an inter-layer insulating layer becomes rough conforming to the rugged shape of device elements and/or wirings. The roughness of the surface causes a deterioration of the dimensional precision when forming an upper-layer interconnect, by using, for example, a photo-lithography process.
In recent years, the intervals between adjacent interconnects, which are formed in semiconductor devices, have become shorter, and multiple-layered interconnects have become commonplace. Accordingly, an evenness of the surface of the semiconductor substrate, on/or above which an interconnecting layer, a device, an under-layer interconnect, and an inter-layer insulating layer are formed, is indispensable. For example, the conventional method of smoothing the above surface by using a process of filling any concave region of the surface of an inter-layer insulating layer with a fluid coating layer, such as the SOG (Spin-on-Glass) coating process, does not comply with the demands for the evenness of the above surface. Accordingly, the CMP (Chemical Mechanical Polishing) process has recently been used; where the inter-layer insulating layer, which is a base for the formation of an interconnect, is chemically and mechanically polished, so as to form an even surface.
The CMP process has a higher capability of removing the bumps of the surface, so as to smooth it, than those of the conventional smoothing processes. However, the CMP process has a somewhat low controllability. This emanates from the "age" deterioration of the surface of a polishing pad due to aging, which is utilized in the CMP process; and/or the influence of a dressing process. Specifically, the age deterioration, such as changes in the polishing grain concentration and/or elastic modulus, on the surface of a polishing pad may occur. This is dependent upon, for example, how the polishing pad has been preserved. In addition, where a dressing process does not fit with a given polishing requirement, the evenness of the surface of the polishing pad may get worse, causing a change in the polishing rate. For correction of the change in the polishing rate, the surface of the polishing pad will need adjusting. According to the conventional method of adjusting the surface of a polishing pad, at first, the surface of the polishing pad is subjected to a dressing process by a dresser, on the surface of which diamond particles are buried by plated nickel. Dressing is sometimes called "seasoning". At the same time or thereafter, a dummy wafer with the surface layer, formed on the surface of the silicon substrate, is utilized to polish the surface of the polishing pad. Wherein, the surface layer of the dummy wafer formed, is made of a material, which is identical to one making up the surface layer of a product wafer to be polished. The adjusting wafer (dummy wafer), used for adjusting the surface of the polishing pad, is fabricated by forming an oxide layer or a metal layer on the surface of a silicon substrate. However, since the adjusting wafer can be used only once or twice, generally, (which though, is dependent upon the thickness of the layer) it is necessary to reform the layer or use another adjusting wafer. The limitation in the case of using a single adjusting wafer, comes from the fact that when the Si surface of the substrate of the adjusting wafer is exposed, it causes a change in the polishing rate. The change in the polishing rate emanates from the difference in the polishing properties between the surface layer of a wafer product, and the silicon substrate of the adjusting wafer.
A polishing pad, made of, in particular, polyurethane foam, generally needs seasoning, right after being replaced. This emanates from the fact, that the probability is high that the polishing rate changes; and that a scratch occurs on a surface layer of a wafer, which has been polished, within twenty to thirty polishing-runs, after the polishing pad has been replaced. Accordingly, it is necessary to polish using twenty to thirty dummy wafers for seasoning, right after the polishing pad has been replaced.
Thus, for the seasoning process, extra adjusting wafers (dummy wafers), are required. In addition, the polishing pad has to be subjected to the seasoning process several times by using multiple, adjusting wafers. This is because the amount shaved by polishing has to be controlled below a certain value, so as to prevent the silicon substrate of each adjusting wafer from being exposed. This causes an occurrence of a problem, that extra time has to be spent for transmitting adjusting wafers and a possible increase in the loss of time.
FIG. 1 is a plan view showing an outline of the conventional polishing apparatus.
A polishing pad 2 is attached to the top of a circular platen 1. The platen 1 is rotated on its own vertical and central axis, with the help of a rotating force, transmitted via a rotating drive shaft, (not shown in the Figure) which is fixed under the platen 1. A carrier 3, which holds a semiconductor wafer or an object to be polished, is placed above the platen 1. The carrier 3 is rotated on its own vertical and central axis, with the help of a spindle mechanism supported by a polishing arm 8; and is placed in such a manner that the under-surface of the semiconductor wafer, held by the carrier 3, can face the polishing pad 2. That is to say, a semiconductor wafer is attached to the carrier 3, in such a manner that the to be polished surface of the semiconductor wafer can face the polishing pad. The carrier 3 is next lowered, onto the surface of the rotating platen 1, and a load is added. Thereafter, the carrier 3 is rotated, in the same direction as that of the platen 1, while an abrasive is supplied to the surface of the polishing pad, thereby polishing the semiconductor wafer. Furthermore, a dressing mechanism, comprised of a dresser 7, is placed beside the platen 1. With this dressing mechanism 6, the polishing pad is dressed, as the semiconductor wafer is being polished or during a break in the polishing processes.
The polishing arm 8 is revolvable, so the carrier 3 can be horizontally moved. A loading cup 10 and an unloading cup 11 are both fixed within the path range, where the carrier 3 moves and reaches. A cleaning platen 13 is also fixed within the path range. A loader 9 and an unloading unit 12 are both placed near the respective loading cup 10 and unloading cup 11.
FIG. 2 is a flowchart showing an example of the conventional polishing steps, including both the steps of adjusting the surface of the polishing pad and the steps of polishing a wafer product.
After the polishing pad has been replaced with a new one, the surface of the polishing pad is subjected to a dressing process for approximately twenty minutes, which is performed by the dresser 7, with the platen 1 is rotated. Consequently, the surface of the polishing pad 2 is scraped by more than several microns in depth. A cassette which stores a plurality of adjusting semiconductor wafers is set on the loader 9 and one of the adjusting semiconductor wafers is placed on the load cup 10. And then, the adjusting semiconductor wafer is mounted on the carrier 3. It is then polished for, for example, three minutes to adjust the pad surface Wherein, the adjusting semiconductor wafer has a silicon oxide layer of 1 micron, in thickness, on its surface. Thereafter, the adjusting semiconductor wafer, is moved into the unloading unit 12 via the unloading cup 11. In the same manner as described above, the polishing of the adjusting semiconductor wafer is repeated twenty times, as an example. Next, a measuring wafer, which is used to measure the polishing rate, is polished only once; and the polishing rate is measured. Thereafter, several tens of wafer products are polished under a certain condition. The adjusting wafer, is polished next, two runs or so, thereby adjusting the surface of the polishing pad. Next, the measuring wafer is polished again, thereby measuring the polishing rate. Several tens of wafer products are polished next, under a certain condition. Thereafter, the same process steps, ranging from the step of polishing the surface of the polishing pad so as to adjust it, to the step of measuring the polishing rate and polishing the wafer products, is repeated.
It is noted that after several tens of wafer products have been polished, polishing process for adjusting the polishing pad is also performed, when no polishing process has been performed on wafer products for more than a fixed period of time; or when it becomes necessary to radically change the polishing condition.
The number of necessary adjusting wafers, which are used in the above process steps, fall within the range of 5% to 20% of the total number of wafer products, processed. The number also depends upon the life span of the polishing pad 2.
As described above, the conventional polishing apparatus and the conventional method of adjusting the surface of the polishing pad 2 both have several problems. The first problem is that the conventional polishing apparatus has a low rate of operation in the polishing process for wafer products. The second problem is that the cost of the conventional polishing apparatus is high, due to the fact that it is necessary to prepare wafers, designated to be used to adjust the surface of the polishing pad. This also causes an increase in the load on the other manufacturing process steps. The last problem is that it is difficult to develop a fully-automated polishing apparatus due to constraints, relevant to the above causes. In particular, it is difficult to develop a fully-automated polishing apparatus, which needs to use a lot of adjusting wafers, due to the fact that an exclusive storage unit for the adjusting wafers is a must; and the number of necessary cassettes, for a wafer supplying unit (not shown in Figures), is high. Further, it is needed to control the cassettes. Therefore, it is difficult to construct an automated polishing apparatus.
In the Japanese Patent Application Laid-open No. Hei-8-148453, the configuration of a wafer holding unit has been disclosed. Wherein, the wafer holding unit is comprised of: a retainer, on the surface of which diamond particles are buried; and a wafer which is to be polished. The wafer is polished by a polishing pad, and at the same time the polishing pad is seasoned by the diamond particles. Due to the configuration of the retainer with diamond particles, the life span of the polishing pad can be expanded. Further, in the above document, a technique for preventing a periphery of a wafer changing the shape is mentioned as a prior art. However, it is not disclosed to an adjusting a surface of a polishing pad.
Furthermore, in the Japanese Patent Application Laid-open No. Sho-64-51267, the configuration of a seasoning apparatus for a polishing pad has been mentioned. The seasoning apparatus has a configuration, where a seasoning material is mounted on a carrier plate, via a packing material. This configuration allows for an easy replacement of the seasoning material. In addition, since the packing material functions as a buffer material, the pressure put onto the polishing pad can be adjusted. In this specification, however, the means for expanding the life span of an adjusting material, which is used to polish the polishing pad, has not been disclosed.